Satisfying the world's growing demand for energy is one of the most significant challenges facing society. At present, about 85% of the energy produced in the United States comes from fossil fuels. Given that the supply of such fuels is on the decline, their prices continue to rise, and the resultant greenhouse gases may contribute to global warming, there is a need to develop new technologies that are economically feasible and environmentally friendly.
Solar energy is one technology for power generation that is clean, quiet and renewable. It is also plentiful: with an average of roughly 125,000 terawatts of solar energy reaching the planet at any given time, solar technology can potentially generate a significant amount of energy.
Solar cells are used to convert solar or radiant energy into electricity. Historically, solar power (both in space and terrestrially) has been predominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency III-V multijunction solar cells has enabled the consideration of this alternative technology for terrestrial power generation. Compared to Si, III-V multijunction cells are generally more radiation resistant and have greater energy conversion efficiencies, but they tend to cost more. Some current III-V multijunction cells have energy efficiencies that exceed 27%, whereas silicon technologies generally reach only about 17% efficiency. Under concentration, some current III-V multijunction cells have energy efficiencies that exceed 37%. When the need for very high power or smaller solar arrays are paramount in a spacecraft or other solar energy system, multijunction cells are often used instead of, or in hybrid combinations with, Si-based cells to reduce the array size.
Generally speaking, the multijunction cells are of n-on-p polarity and are composed of InGaP/(In)GaAs/Ge compounds. III-V compound semiconductor multijunction solar cell layers can be grown via metal-organic chemical vapor deposition (MOCVD) on Ge substrates. The use of the Ge substrate permits a junction to be formed between n- and p-Ge. The solar cell structures can be grown on 100-mm diameter (4 inch) Ge substrates with an average mass density of about 86 mg/cm.sup.2. In some processes, the epitaxial layer uniformity across a platter that holds 12 or 13 Ge substrates during the MOCVD growth process is better than 99.5%. Each wafer typically yields two large-area solar cells. The cell areas that are processed for production typically range from 26.6 to 32.4 cm2. The epi-wafers can be processed into complete devices through automated robotic photolithography, metallization, chemical cleaning and etching, antireflection (AR) coating, dicing, and testing processes. The n- & p-contact metallization is typically comprised of predominately Ag with a thin Au cap layer to protect the Ag from oxidation. The AR coating is a dual-layer TiOx/Al2O3 dielectric stack, whose spectral reflectivity characteristics are designed to minimize reflection at the coverglass-interconnect-cell (CIC) or solar cell assembly (SCA) level, as well as, maximizing the end-of-life (EOL) performance of the cells.
In some multifunction cells, the middle cell is an InGaAs cell as opposed to a GaAs cell. The indium concentration may be in the range of about 1.5% for the InGaAs middle cell. In some implementations, such an arrangement exhibits increased efficiency. The InGaAs layers are substantially perfectly lattice-matched to the Ge substrate.
Regardless of the type of cell used, a known problem with solar energy systems is that individual solar cells can become damaged or shadowed by an obstruction. For example, damage can occur as a result of exposure of a solar cell to harsh environmental conditions. The current-carrying capacity of a panel having one or more damaged or shadowed solar cells is reduced, and the output from other panels in series with that panel reverse biases the damaged or shadowed cells. The voltage across the damaged or shadowed cells thus increases in a reverse polarity until the full output voltage of all of the panels in the series is applied to the damaged or shadowed cells in the panel concerned. This causes the damaged or shadowed cells to breakdown.
As a solar cell system for terrestrial applications has thousands of solar cells, its voltage output is normally in the range of hundreds of volts, and its current output is in the range of tens of amperes. At these output power levels, if the solar cell terminals are not protected, uncontrollable electric discharge in the form of sparks tends to occur, and this can cause damage to the solar cells and to the entire system.